This seemed to be the first I had heard of this format… being lewd, however, I seem to have already established a few in my browsing history. I found this out when I typed "gifv" into my address bar to learn more… anyhow, a quick peak at the source code gave me this snippet.
<a class="" title="Greetings GIF Creators! (HUGE)" href="http://i.imgur.com/zvATqgs.gifv">
   <video autoplay="" loop="" muted="">
      <source type="video/webm" src="http://i.imgur.com/zvATqgs.webm">
      <source type="video/mp4" src="http://i.imgur.com/zvATqgs.mp4">
   </video>
</a>
If we're talking about forum posts or chat messages, seems to me a simple regexp should do the trick.
~i.imgur.com/(\w+)\.gifv~
~i.imgur.com/$1.webm~
From there just wrap it in a video tag and kick back. This could be implemented as JS or server-side.